This invention relates to output drivers and, in particular, to BiCMOS TTL output drivers having independent DC drive and transient response controls.
A typical TTL output driver has push and pull transistors both coupled to an output for respectively providing a logic high and a logic low voltage level at the output. The push and pull transistors are typically coupled to a control circuit for alternately rendering the push and pull transistors operative in response to an input logic signal. The pull transistor of a typical TTL output driver, when operative, usually must sink a predetermined DC drive current since it is typically coupled to a TTL input circuit which will source a predetermined current when pulled into a logic low state. Further, as the DC drive current through the pull transistor increases, the transition rate (transient response) from a logic high voltage level to a logic low voltage level also increases. Therefore, a large DC drive corresponds to a fast transient response while a small DC drive corresponds to a slow transient response. However, some applications may require a small DC drive with a fast transient response, or even a large DC drive with a slow transient response. Therefore, it would be advantageous to provide a circuit to control the transient response independent of the DC drive.
Hence, a need exists for an output driver circuit having independent DC drive and transient response controls.